For the 500 series, the easiest route is to get an extended mag tube and an 8 shot barrel. That's what I did on my last extension. I also found an 8-shot 24" or 26" V.R. factory barrel. I did build an extended tube 500 years ago using a 5500 mag extension but that requires a LOT of work to make it work right, including knocking the lug off the barrel!